1. Field of the Invention:
The present invention relates to a method and apparatus for disintegrating used paper into fluffy pulp fibers in accordance with a dry process.
2. Description of the Prior Art:
Heretofore, as an apparatus for dry disintegration of used paper there has been proposed such an apparatus as illustrated in FIGS. 19 to 21. This conventional used paper disintegrating apparatus is provided with a circular casing body 110 and an impeller 120, the circular casing body 110 having an inlet port 111 and an outlet port 112 and further having undulating teeth 114 which are formed circumferentially on the inner peripheral surface of the circular casing body and are undulating in section, and the impeller 120 being mounted coaxially within the circular casing body 110 through a clearance formed between the impeller and the undulating teeth 114. The inlet port 111 and the outlet port 112 are open tangentially in the peripheral surface of the circular casing body 110.
In the above conventional apparatus, by a current of air generated with rotation of the impeller 120, finely cut pieces of used paper M are sucked in from the inlet port 111 together with the air current. The used paper pieces M thus sucked in are caught by the undulating teeth 114 as shown in FIG. 20 under the action of a rotating current which is generated by the rotation of the impeller 120. One ends of the used paper pieces M are retained by the undulating teeth 114, while the opposite ends thereof are beaten by-blades 122 of the impeller 120 which are rotating at high speed, whereby the used paper is disintegrated while being torn off.
In connection with the above conventional disintegration principle, various other explanations have so far been made such as, for example, an explanation to the effect that the used paper pieces M are disintegrated by ultrasonic vibration using a high-speed air current, and an explanation to the effect that the used paper pieces strike against the undulating teeth or roll thereon and are thereby disintegrated. However, according to a phenomenon observed most frequently in ultra-high speed photographing, a large number of used paper pieces M are sandwiched in between the addendum tips of teeth 114 and the distal ends of the blades 122 of the impeller 120 and are torn or ground simultaneously for disintegration, as shown in FIG. 21, (such a way of disintegration involving tearing or grinding of used paper pieces M and the foregoing disintegration method involving tearing of used paper pieces will hereinafter be referred to genetically as "mechanical disintegration").
According to the conventional apparatus for dry integration of used paper described above, however, about 30% to 50% or more in weight ratio of powder is incorporated in the disintegrated pulp fibers of used paper, and thus the amount of powder produced is large. Besides, the length of the disintegrated pulp fibers is short.
Such pulp fibers of used paper with a large amount of powder incorporated therein are an obstacle to their subsequent re-utilization. For example, it is known that if they are used as the starting material of recycled paper, a marked deterioration of the paper toughness will result. This problem my be solved by separating and removing the powder after disintegration. However, in the disintegrated used paper, pulp fibers are in a complicatedly entangled state like fluff and the powder is caught by such entangled pulp fibers, so it cannot easily be separated and removed.
The present inventors have made studies for finding out conditions to minimize the formation of powder in the conventional apparatus and thereby solve the abovementioned problems while varying the number of revolutions of the impeller 120, the size and shape of used paper pieces M, the amount thereof to be fed, the velocity of air current (amount of air), sectional shape of the undulating teeth 114, and the clearance between the addenda of the teeth 114 and the outer peripheral edge of the impeller member 120. As a result, we found out that under a certain condition a high proportion of used paper pieces M came to have a predetermined rigidity like wooden pieces while being conveyed pneumatically with a current of air generated by the rotation of the impeller 120 and came into collision with the undulating teeth 114, resulting in the collided portions being beaten successively or the whole thereof being disintegrated almost instantaneously (hereinafter referred to as "pneumatic disintegration" in contrast with mechanical disintegration) with scarcely any contact of the used paper pieces M with the impeller 120, that is, unlike the mechanical disintegration involving cutting or grinding of the used paper pieces.
The used paper pieces M, not coming into contact with the impeller 120, strike against slant faces of the undulating teeth 114 and are disintegrated thereby while being conveyed pneumatically by a current of air. The pulp fibers of the used paper pieces thus disintegrated were longer in average fiber length and much smaller in the amount of powder produced in comparison with those disintegrated mechanically.
When in view of the above test results the clearance between the addenda of the undulating teeth 114 and the outer peripheral edge of the impeller 120 was enlarged to not less than 1 mm in order to minimize the influence of the impeller 120, it was possible to make sure that the proportion of powder incorporated in the disintegrated pulp fibers was high and that the proportion of long fibers in the disintegrated pulp fibers was very high. In other words, it turned out that the conventional apparatus involved the problem of disintegrated pulp fibers being short and the amount of powder produced large because of mechanical disintegration of finely cut used paper pieces.